From considerations of the equilibrium of the subpleural alveoli, we concluded that the measurement of a single ratio, the circumference of a subpleural alveolus to its area, associated with the measurement of the pressure-volume relationships for air and saline inflation permit us to compute the surface tension for the surfactant lining in situ. Conventional surface tension masurements, obtained with lung extract, differ considerably from our mesurements. We postulate that these differences are due to changes in the interface between the in vivo and the in vitro state of the surface. Because of the important role of the surface tension in the respiratory distress syndrome, (RDS) it is the objective of our first project to employ our newer and more precise formulation to measure the in situ surface tension in the lungs of fetus and newborn rabbits. The change of a quantitative index, selected from this measurement can then be used for evaluating the maturity of the lung and the effectiveness of a therapeutic method for RDS. In the second project, a perfusion system will be used to induce acute edema in an isolated lobe of the dog's lung. With a microscope system, we shall examine the fluid accumulation in the septa and the air space of the individual alveoli. Using a mechanical and filtration model, we shall analyze the time sequence of the accumulation and its correlation wth the change in lung weight. It is our aim to identify the role of surfactant in the development of edema. With a lobe set up in a more rigorously controlled perfusion system, a critical examination of the present injecting and sampling technique for the indicator dilution technique is proposed in the third project. Together with the results obtained from the second project, it is our objective to investigate the accuracy of the double-indicator- dilution technique for early detection of pulmonary edema and to modify it appropriately if necessary.